In industrialized societies, large amounts of energy are consumed to make up the thermal losses occurring in a wide variety of applications due to poor thermal insulation. For example, in applications involving thermal cycling, thermal insulation is critical to increase system efficiency. In various other applications, thermal insulation is critical to maintain microenvironment temperature. Thus, for example effective thermal insulation reduces the energy expenditure required to heat or cool buildings or appliances such as ovens and freezers.
Silicon oxide (silica) aerogel, basically a glass matrix comprising nano to micro voids, has been proven to be a good thermal insulator, especially for applications at high temperatures and in cases where non-flammable insulators are important. However, due to the high cost and limited insulation performance in large-scale industrial production, aerogels have not been widely used as thermal insulators.
One of the problems that has limited the performance of aerogels as thermal insulators arises from the conflicting requirements necessary to keep both the blackbody radiation and the solid thermal conductivity low. By way of example, for a homogenous silica aerogel insulation layer, the total thermal energy loss (Qeff) through the insulation layer is consists of three components: 1) The thermal loss via gas thermal conduction (Qg); 2) The radiation thermal loss (Qr); and 3) The thermal loss via solid thermal conduction (Qs), as described in Eq. 1, below:Qeff=Qg+Qr+Qs  (Eq. 1)where for a slab structured insulation layer,
                    Q        =                                            A              ⁡                              (                                                      T                    1                                    -                                      T                    2                                                  )                                      ⁢            λ                    δ                                    (                  Eq          .                                          ⁢          2                )            and for a cylinder structured insulation layer:
                    Q        =                              2            ⁢            π            ⁢                                                  ⁢                          λ              ⁡                              (                                                      T                    1                                    -                                      T                    2                                                  )                                                          ln            ⁡                          (                                                d                  1                                                  d                  2                                            )                                                          (                  Eq          .                                          ⁢          3                )            where A is the area that the heat flux pass through, T1 and T2 are the temperatures at two sides of the insulator surfaces, δ is the thickness of the insulation material d1 and d2 are the diameters of the cylinder structured homogenous silica aerogel insulation layer, and the λ is the total thermal conductivity that is given by the following equation,λeff=λg+λr+λs  (Eq. 4)where λeff is the total effective thermal conductivity of the aerogel, in W/(m·K) which consists of three terms. The first term λg is the contribution to the thermal conductivity from the gas and is given by the following equation:
                              λ          g                =                              λ                          g              ⁢                                                          ⁢              0                                            (                          1              +                                                2.3                  ×                                      10                    4                                                                    ϕ                  ⁢                                                                          ⁢                  p                                                      )                                              (                  Eq          .                                          ⁢          5                )            where λg0 is the thermal conductivity of the still gas (air) in W/(m·K) since the mean free pass of the gas molecules in silica aerogel is severely limited by the nano or micro voids structure, ϕ is the parameter that characterizes this limitation, in μm; P is the pressure, in Pa (see, e.g., Lee et al. (2002) J. Non-Crystalline Solids 298: 287-292).
According to the equation of radiative flux
      Q    r    =            A      ⁢                          ⁢              σ        ⁡                  (                                    T              1              4                        -                          T              2              4                                )                                    3        ⁢                                  ⁢        β        ⁢                                  ⁢                  δ          /          4                    +              (                              1            /                          ɛ              1                                +                      1            /                          ɛ              2                                -          1                )            (see, e.g., Howell et al. Thermal Radiation Heat Transfer[M], Fifth Edition, Pp. 591-595) and the relation
            Q      r        =                            A          ⁡                      (                                          T                1                            -                              T                2                                      )                          ⁢                  λ          r                    δ        ,the conductivity contributed by radiation λr can be obtained as
                              λ          r                =                                            σ              ⁡                              (                                                      T                    1                    2                                    +                                      T                    2                    2                                                  )                                      ⁢                          (                                                T                  1                                +                                  T                  2                                            )                        ⁢            δ                                              3              ⁢                                                          ⁢              β              ⁢                                                          ⁢                              δ                /                4                                      +                          (                                                1                  /                                      ɛ                    1                                                  +                                  1                  /                                      ɛ                    2                                                  -                1                            )                                                          (                  Eq          .                                          ⁢          6                )            
The second term λr describes the thermal loss contribution from the blackbody radiation, where σ is the Stefan-Boltzmann constant (σ=5.67×10−8 W/(m2 K)); δ is the thickness of the insulation aerogel material, T1 is the cold side temperature of the aerogel and T2 is the hot side temperature of the aerogel, in K; ε is the emissivity of the aerogel; and β is the attenuation coefficient (extinction coefficient) of the aerogel, in m−1.
The third term λs is the contribution from the solid thermal conduction of the silica aerogel and is given by the following equationλs=cρb  (Eq. 7)(see, e.g., Zeng et al. (1995) J. Non-Crystal Solid, 186: 271-277), where c is a constant that is independent of the aerogel density and the scaling exponent; ρ is the density of the aerogel, in kg/m3; b is a scaling exponent and depends on the aerogel structure, but typically has a value of 2 (see, e.g., Lu et al. (1992) Science 255: 971).
According to Eq. 7, one can see that the main advantage of the silica aerogel in the application of thermal insulation is that at very low density it can dramatically reduce the solid thermal conduction. This is why aerogels typically provide better insulation properties than other insulation materials.
However, as one decreases the first term and the third term contribution by reducing the silica aerogel density and the insulation layer air pressure, the second term λr, i.e., the blackbody radiation contribution to the effective thermal conduction becomes important, especially at higher temperature T2 (e.g., when T2>>ambient temperature, in K). As indicated by the second term, contribution from the blackbody radiation is to be reduced, the attenuation coefficient β, the total absorption and scattering of the radiation by the aerogel is increased. However, this is in contradiction to the need to decrease the density of the aerogel to reduce the third term, as lower density normally causes lower absorption.
Accordingly, other absorption agents have been added into the to maintain fairly high density and balance the contributions from both the blackbody radiation and solid thermal conduction (see, e.g., Zeng et al. (1995) J. Non-Crystalline Solids, 186: 271-277).
One can see from modeling of carbon content to minimize heat transfer in silica aerogels (Id.) that a density of ˜160 kg/m3 (˜8%) is believed to provide the best overall insulation performance for the silica aerogel and this is the density typically utilized by aerogel manufactures. As noted in Zeng et al. supra., carbon was added into the aerogel and it was found that at ambient temperature, adding approximately 8% carbon into a silica aerogel could lower the total thermal energy loss by about ⅓.
At temperature as high as 600K, the transparent aerogel has 10 times larger total thermal conductivity than that of an opaque aerogel using optimized carbon content. In addition, the optimal carbon content that minimizes the overall thermal conductivity increases with the working temperature. For example at 600K, an aerogel containing about 16% carbon provides the optimal overall insulation performance. However, adding absorption agents actually increases the solid thermal conduction contribution and complicates the manufacturing process (Id.).
According to Eq. 5, the thermal conductivity of gas molecules (air) in silicon oxide aerogels is reduced by the void structures and characterized by the parameter Φ. As described above, the second term contribution may be relatively high at higher working temperatures. Hence, there is no practical value to further reduce the contribution of the first term by reducing gas density (vacuum treatment for the aerogel insulation applications). As a consequence, thermal insulation properties of industrial aerogel have been limited to about 3 times better than other conventional thermal insulators, e.g., to be about 0.02 W/mK.
One of the main costs in the production of silica aerogels has been the requirement for pure ethanol (e.g., ethanol concentration typically better than 99%). IN most approaches for the fabrication of silica aerogels, a gel precursor is formed that mainly contains pure ethanol and organic silicon. Subsequently the ethanol is eliminated from the gel by a supercritical drying process of either ethanol or carbon dioxide/ethanol. Since the ethanol evaporated in the drying process contains a certain amount of water, this ethanol is either typically vented and disposed of as waste or it has to be distilled before reuse. As a consequence, due to the high cost of ethanol and/or ethanol distillation silica aerogels have, to date, been characterized by a high manufacturing cost.
In addition, in previous methods of aerogel fabrication that involve supercritical drying of ethanol, because the ethanol volume expands about 3 times before reaching the supercritical temperature, normally only about ⅓ of the volume of the supercritical drying vessel is filled with gel because the gel has to be completely emerged in the pure ethanol liquid. Moreover, supercritical drying vessels for ethanol are very expensive because they must be built to contain a highly combustible fluid at high temperature and pressure. Because only about ⅓ of the supercritical drying vessel is filled, the utilization rate of that vessel is quite low. Thus the equipment cost is also relatively high for traditional aerogel manufacturing processes.
Additionally packaging of SiO2 aerogel, for example, into insulating structures, has also proven difficult. Since pure aerogels are very brittle, fiber-glass or other materials have been used as matrix materials to contain the SiO2 aerogel. During the installation processes aerogel powders often fall out the matrix, again leading to poor aerogel utilization.